feat: add confidence scoring

worker/lyra_worker/confidence.py

@@ -0,0 +1,21 @@
+from difflib import SequenceMatcher
+
+from lyra_worker.types import Candidate, MBTarget
+
+
+def _sim(a: str, b: str) -> float:
+    return SequenceMatcher(None, a.strip().casefold(), b.strip().casefold()).ratio()
+
+
+def score_confidence(target: MBTarget, c: Candidate) -> float:
+    """0..1 confidence that candidate c is the requested target."""
+    artist_sim = _sim(target.artist, c.matched_artist)
+    album_sim = _sim(target.album, c.matched_album)
+
+    if target.track_count is None or target.track_count == c.track_count:
+        track_factor = 1.0
+    else:
+        diff = abs(target.track_count - c.track_count)
+        track_factor = max(0.0, 1.0 - diff / max(target.track_count, 1))
+
+    return 0.5 * artist_sim + 0.4 * album_sim + 0.1 * track_factor